A synchrotron self-Compton model with low-energy electron cut-off for the blazar S5 0716+714

Context.In a self-absorbed synchrotron source with power-law electrons, rapid inverse Compton cooling sets in when the brightness temperature of the source reaches $T_{\rm B}\sim$1012 K. However, brightness temperatures inferred from observations of intra-day variable sources (IDV) are well above the “Compton catastrophe” limit. This can be understood if the underlying electron distribution cuts off at low energy. Aims.We examine the compatibility of the synchrotron and inverse Compton emission of an electron distribution with low-energy cut-off with that of IDV sources, using the observed spectral energy distribution of S5 0716+714 as an example. Methods.We compute the synchrotron self-Compton (SSC) spectrum of monoenergetic electrons and compare it to the observed spectral energy distribution (SED) of S5 0716+714. The hard radio spectrum is well-fitted by this model, and the optical data can be accommodated by a power-law extension to the electron spectrum. We therefore examine the scenario of an injection of electrons, which is a double power law in energy, with a hard low-energy component that does not contribute to the synchrotron opacity. Results.We show that the double power-law injection model is in good agreement with the observed SED of S5 0716+714. For intrinsic variability, we find that a Doppler factor of ${\cal D}$≥30 can explain the observed SED provided that low-frequency (